The present invention relates to a load-sensing hydraulic drive circuit for hydraulic machines, such as hydraulic excavators and cranes, each equipped with a plurality of hydraulic actuators, and more particularly to a control system for a load-sensing hydraulic drive circuit, which is designed to control the flow rates of hydraulic fluid supplied to the hydraulic actuators using pressure compensated flow control valves, while holding the delivery pressure of a hydraulic pump higher by a predetermined value than the maximum load pressure among the hydraulic actuators.
In these days, a load-sensing hydraulic drive circuit has been employed in hydraulic machines, such as hydraulic excavators and cranes, each equipped with a plurality of hydraulic actuators.
The hydraulic drive circuit comprises a pressure compensated flow control valve connected between a hydraulic pump and each of the hydraulic actuators for controlling the flow rate of hydraulic fluid supplied to the hydraulic actuator in response to an operation signal from a control lever, and a load-sensing regulator for holding the delivery pressure of the hydraulic pump higher by a predetermined value than the maximum load pressure among the plural hydraulic actuators. The pressure compensated flow control valve has a pressure compensating function to maintain the flow rate constant regardless of fluctuations in the load pressure or the delivery pressure of the hydraulic pump, so that a flow rate proportional to the operated amount of each control lever is supplied to the associated hydraulic actuator. As a result, independent operations of the respective hydraulic actuators are ensured when a plurality of hydraulic actuators are operated in a combined manner. The load-sensing regulator functions to constantly maintain the delivery pressure of the hydraulic pump at a lower limit corresponding to the maximum load pressure among the hydraulic actuators for energy saving.
However, the above load-sensing hydraulic drive circuit has the following problem which is specific to load-sensing control. The delivery amount of a variable displacement hydraulic pump is determined by the product of its displacement, i.e., inclination angle of a swash plate, in the case of a swash plate type and the rotational speed of the pump. The larger the inclination angle of the swash plate, the larger the delivery amount of the pump. The inclination angle of the swash plate has an upper limit determined by the pump structure, at which upper limit of the delivery amount of the pump also reaches its maximum. But, the pump is driven by a prime mover, and if input torque of the pump exceeds output torque of the prime mover, the rotational speed of the prime mover would be reduced and even lost in the worst case. To avoid such an event, an input torque regulator has usually been equipped on the pump to limit the maximum inclination angle of the swash plate so that input torque of the pump will not exceed output torque of the prime mover, thereby controlling the delivery amount of the pump input torque limiting control.
When the total of demand flow rates for the plural actuators commanded by the respective control levers exceeds the available maximum delivery amount of the pump during combined operation of the actuators, the pump cannot increase the delivery amount (inclination angle) much more even though it is under the load-sensing control. In other words, the delivery amount of the pump is saturated. As a result, the delivery pressure of the pump is reduced and can no longer be maintained higher by a predetermined value than the maximum load pressure. Thus, the delivery amount of the pump is caused to largely flow into the actuator(s) on the lower pressure side, while the hydraulic fluid is not supplied to the actuator(s) on the higher pressure side, resulting in a problem that the combined operation of plural actuators cannot be performed smoothly.
To solve the above-mentioned problem, DE-A1-3422165 (corresponding to Japanese Patent Laid-Open No. 60-11706) has proposed such a circuit arrangement that a pair of opposing pilot chambers is added to a pressure balance valve of each pressure compensated flow control valve, and the delivery pressure of the pump is introduced to one of the pilot chambers which acts in the valve-opening direction, while the maximum load pressure among the plural actuators is introduced to the other pilot chamber which acts in the valve-closing direction. With the circuit arrangement, when the total of demanded flow rates for the plural actuators commanded by the respective control levers exceeds the maximum delivery amount of the pump, throttle openings of the respective pressure balance valves are reduced at the same proportion as each other in accordance with a reduction in the delivery pressure of the pump, so that the flow rates through the respective flow control valves are restricted in a manner corresponding to the ratios of throttle openings (demand flow rates) of the flow control valves. Therefore, the hydraulic fluid is reliably supplied to the actuator(s) on the higher pressure side as well, for achieving the combined operation with certainty.
The pressure compensated flow control valve determines a consumable flow rate, that is to be passed to the associated hydraulic actuator therethrough, based on both a throttle opening command value for the flow control valve given by an operation signal from the control lever and a differential pressure command value across the flow control valve given to the pressure balance valve. Both the throttle openings of the flow control valve and the pressure balance valve are controlled so that the actual flow rate through the pressure compensated flow control valve, i.e., the flow rate consumed by the actuator becomes equal to the consumable flow rate. In the above prior art, the differential pressure command value across the flow control valve is directly applied to the pressure balance valve hydraulically such that the delivery pressure of the pump and the maximum load pressure among the hydraulic actuators are introduced to the pressure balance valve in opposite directions, causing the differential pressure therebetween to act on the pressure balance valve. By so doing, the differential pressure command values applied to all the pressure balance valves are limited to compensate (reduce) the total consumable flow rate for all the hydraulic actuators. This reduces the total flow rate actually consumed by the actuators. Hereinafter, this type of control will be referred to as total consumable flow compensating control. It is to be noted that, in the total consumable flow compensating control in the above prior art, the differential pressure between the pump delivery pressure and the maximum load pressure is reduced responsive to deficiencies in the actual delivery pressure of the pump as compared with the demand flow rates commanded by the control levers, and hence, the total consumable flow rate is always coincident with the total of actual flow rates consumed by the hydraulic actuators.
In the foregoing prior art, because the pressure compensated flow control valve is controlled to be directly responsive to the differential pressure between the pump delivery pressure and the maximum load pressure for carrying out the total consumable flow compensating control, the load-sensing control of the pump and the total consumable flow compensating control of the pressure compensated flow control valve are concurrently controlled when the delivery pressure of the pump is reduced. This has accompanied the problem below.
More specifically, the load-sensing control controls the delivery amount of the pump to hold the differential pressure constant, and has a slower response speed than that of the total consumable flow compensating control, as the control of the delivery amount of the pump is carried out through various mechanisms. Therefore, when the delivery pressure of the pump is reduced at the moment the control lever is operated to start supply of the hydraulic fluid to the actuator or increase the supply amount thereof, the flow rate through the pressure compensated flow control valve starts to be restricted under the total consumable flow compensating control before the load-sensing control starts to increase the delivery amount of the pump. This causes the problem that in a transitional period, the flow rate supplied to the actuator cannot be increased and the operability is impaired even though the control lever is operated with an intention to increase the flow rate.
In a similar case, it may happen repeatedly that the pump delivery amount is increased under the load-sensing control to raise up the pump delivery pressure after the flow rate through the flow control valve has been restricted under the total consumable flow compensating control, then the total consumable flow compensating control is released to increase the flow rate through the flow control valve, causing the delivery pressure of the pump to be reduced, and thereafter the flow rate through the flow control valve is restricted under the total consumable flow compensating control before the load-sensing control has started to increase the pump delivery amount. In other words, the load-sensing control and the total consumable flow compensating control interfere with each other, thereby resulting in a hunting phenomenon.
It is an object of the present invention to provide a control system for a load-sensing hydraulic drive circuit which can perform the total consumable flow compensating control of pressure compensated flow control valves, even in the case when the delivery amount of the pump is saturated, ensuring excellent operability, and offering stable control, free of a hunting phenomenon.